The International Space Station (ISS) is a $100 billion (£80 billion) science and engineering lab located 250 miles (400 km) above the Earth.
Since November 2000, it has been constantly staffed with interchangeable crews of cosmonauts and cosmonauts.
The crews came mainly from the US and Russia, but astronauts were also sent by the Japanese space agency JAXA and the European space agency ESA.
The International Space Station has been in continuous use for over 20 years and has been expanded with many new modules and system upgrades.
Research conducted aboard the ISS often requires one or more of the unusual conditions present in low Earth orbit, such as low gravity or oxygen.
ISS research focuses on human research, space medicine, life sciences, physical sciences, astronomy, and meteorology.
The US space agency, NASA, spends about $3 billion (£2.4 billion) a year on the space station program, with the remaining funding coming from international partners including Europe, Russia and Japan.
So far, 244 people from 19 countries have visited the station, including eight private individuals who have spent up to $50 million on their visit.
There is ongoing debate about the station’s future after 2025, with some of the original design thought to reach “end of life”.
Around the same time, Russia, the station’s main partner, plans to launch its own orbital platform, and the private firm Axiom Space plans to simultaneously send its own modules to the station for purely commercial use.
NASA, ESA, JAXA and the Canadian Space Agency (CSA) are working together to build a space station in orbit around the Moon, while Russia and China are working on a similar project that will also include a surface base.